Light-receiving device and method for producing the same

ABSTRACT

A light-receiving device includes a light-receiving layer having an undoped multi-quantum well structure; a cap layer disposed on the light-receiving layer, the cap layer including a semiconductor layer doped with a p-type impurity; a mesa structure including the cap layer; a p-type region extending from the p-type semiconductor layer toward the light-receiving layer, the p-type region including the p-type impurity diffused from the semiconductor layer in the mesa structure; a p-n junction formed at an end of the p-type region; and an electrode disposed on the cap layer of the mesa structure. The mesa structure is defined by a trench surrounding the mesa. The trench has a bottom that reaches the vicinity of an upper surface of the light-receiving layer. The p-n junction is located in the light-receiving layer or at the boundary between the light-receiving layer and the cap layer disposed on the light-receiving layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of Ser. No. 14/154,861 filed on Jan. 14,2014, which claims priority from JP2013-004879, filed Jan. 15, 2013,which is incorporated in its entirety herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-receiving device and a methodfor producing the same.

2. Description of the Related Art

Living bodies, such as animals and plants, and substances, such as gasesassociated with the environment, have the characteristics of absorbingspecific light in the infrared region including the near-infrared lightwith a wavelength of about 2 μm to 10 μm. In order to detect theabsorption spectra of the substances such as gases associated withenvironments or organisms such as animals and plants, infraredlight-receiving devices and infrared sensing devices including suchinfrared light-receiving devices are under development. In particular,in the near-infrared to infrared region, the photosensitivity tolong-wavelength light is being improved. Here, the infraredlight-receiving devices include a light-receiving layer composed of aIII-V group compound semiconductor. In the light-receiving devices, aplanar-type photodiode structure in which a p-n junction is formed byselective diffusion is used to reduce the dark current. In thisplanar-type photodiode structure, pixel isolation is achieved with aregion in which impurity (for example, Zn impurity) is not diffused.

On the other hand, a mesa-type photodiode is also developed. In themesa-type photodiode, a mesa structure is isolated by a mesa groovearranged surrounding the mesa structure. Non-Patent Literature 1(Marshall J. Cohen; Michael J. Lange; Martin H. Ettenberg; Peter Dixon;Gregory H. Olsen. A Thin Film Indium Gallium Arsenide Focal Plane Arrayfor Visible and Near Infrared Hyperspectral Imaging, Proceedings of theLEOS '99, IEEE Lasers and Electro-Optics Society 1999 12th AnnualMeeting Conference, pp. 744-745) and Patent Literature (JapaneseUnexamined Patent Application Publication No. 2001-144278) reporttwo-dimensional near-infrared InGaAs image sensors including themesa-type photodiodes in contrast with the planar-type photodiodes.Furthermore, Non-Patent Literature 2 (Mark Itzler. Low-Noise AvalanchePhotodiodes for Midwave Infrared (2 to 5 μm) Applications, PrincetonLightwave Inc., Final Report, 14 Aug. 2005) reports a method for formingthe mesa structure including the light-receiving layer having a type-IImulti-quantum well (MQW) structure to extend sensitivity to longerwavelengths from the near-infrared region. Specifically, the Non-PatentLiterature 2 reports wet etching with a phosphoric acid-based etchant toform the mesa structure.

SUMMARY OF THE INVENTION

However, the planar-type photodiode formed by selective diffusion hasproblems described below.

(1) When the light-receiving devices are formed on a large-diameterwafer, production efficiency can be enhanced. However, it is difficultto uniformly perform selective diffusion over the large-diameter wafer.(2) If a region in which impurity is not diffused is not sufficientlyensured, it is difficult to separate pixels. Therefore, the region inwhich impurity is not diffused is formed with a sufficient area betweenthe pixels for reliably separating the pixels. As a result, the areafraction of an opening or selectively diffused region occupying anincident surface (that is, so-called “fill factor”) is reduced. Thus,sensitivity improvement is limited. In another respect, an increase inpixel pitch density is limited.

The mesa-type photodiodes have the advantage of having larger fillfactors than the planar-type photodiodes. The mesa-type photodiodes haveanother advantage of having excellent controllability of positions ofp-n junctions because p-n junctions are formed by epitaxial growth. Thedeviation of positions of p-n junctions changes the dependence ofsensitivity and response speed on bias voltage, thus affecting thestability of properties of the photodiodes.

However, the mesa-type photodiode has a large leakage current that flowson a side surface of a mesa structure because of the exposure of a p-njunction at the side surface of the mesa structure. The leakage currentcauses the dark current of the photodiode. In particular, when the mesastructure is formed by dry etching, crystal damage occurs due to dryetching. The crystal damage due to dry etching tends to cause anincrease in dark current. In general, dark current in the photodiode isrequired to be minimized. In order to reduce the dark current of alight-receiving device (photodiode), the light-receiving device may becooled. Thus, a cooling mechanism, such as a Peltier element, may beprovided to cool the light-receiving device. However, a light-receivingdevice having a cooling mechanism has demerits of increasing the devicesize and producing cost, for example. Therefore, a light-receivingelement having a structure without a cooling mechanism, the structure byitself is a big feature of the light-receiving device. Even if a coolingmechanism is required, a low level of dark current makes it possible toreduce a cooling performance of the cooling mechanism, thereby reducing,for example, the size and cost of the cooling mechanism and thelight-receiving device. It is thus important to form a mesa structure inwhich an edge of a p-n junction is not exposed to the atmosphere inorder to reduce the dark current.

The position of a p-n junction is also important in addition to thenon-exposure of the p-n junction. In general, dark current isproportional to an operating voltage (the absolute value of areverse-bias voltage). Thus, a lower operating voltage results infurther suppression of dark current. To extend a depletion layer from ap-n junction into a light-receiving layer, the operating voltage isapplied between a pixel electrode and a ground electrode. When a p-njunction is provided in a light-receiving layer or at the boundarybetween an upper layer and a light-receiving layer, it is possible toextend a depletion layer into the light-receiving layer at a lowoperating voltage.

A light-receiving device according to a first aspect of the presentinvention includes (a) a light-receiving layer disposed on a substrate,the light-receiving layer having an undoped multi-quantum wellstructure; (b) a cap layer disposed on the light-receiving layer, thecap layer including a p-type semiconductor layer doped with a p-typeimpurity; (c) a mesa structure disposed on the substrate, the mesastructure including the cap layer; (d) a p-type region extending fromthe p-type semiconductor layer toward the light-receiving layer, thep-type region including the p-type impurity diffused from the p-typesemiconductor layer in the cap layer in the mesa structure; (e) a p-njunction formed at an end of the p-type region; and (f) an electrodedisposed on the cap layer of the mesa structure. The mesa structure isdefined by a trench surrounding the mesa. The trench has a bottom thatreaches the vicinity of an upper surface of the light-receiving layer.In addition, the p-n junction is located in the light-receiving layer orat the boundary between the light-receiving layer and the cap layerdisposed on the light-receiving layer.

In the light-receiving device of the first aspect of the presentinvention, the p-n junction is formed at the end of the p-type regionthat is defined by the diffusion of the p-type impurity (Zn) from thecap layer in the mesa structure. In addition, the mesa structure isdefined by the trench surrounding the mesa. Therefore, the p-n junctionis separated using the mesa structure and the p-type region, therebyfacilitating a reduction in the size of the light-receiving device,compared with a planar-type photodiode in the related art in which a p-njunction is formed and separated only by selective diffusion.

Furthermore, the bottom of the trench reaches the vicinity of an uppersurface of the light-receiving layer. The p-n junction is located in thelight-receiving layer or at the boundary between the light-receivinglayer and the cap layer disposed on the light-receiving layer.Therefore, an edge of the p-n junction is not exposed to the atmosphere.In addition, the edge of the p-n junction is not exposed to anatmosphere in a growth chamber even during the production process. Thus,an impurity, such as oxygen, contained in the atmosphere and so forthdoes not adhere to the edge of the p-n junction, thereby suppressing anincrease in dark current.

In addition, the p-n junction is located near the light-receiving layer.It is thus possible to extend a depletion layer to the light-receivinglayer even at a low operating voltage. Hence, a lower operating voltageresults in lower dark current.

The distribution of impurity concentration is controlled by the dopingconcentration of the p-type semiconductor layer in the cap layer. It isthus possible to increase the size of a substrate, compared with aselective diffusion method in the related art in which the selectivediffusion is performed in a silica tube, for example. This enhances themass productivity of the light-receiving device.

In the light-receiving device, the bottom of the trench reaches thevicinity of an upper surface of the light-receiving layer as mentionedabove. That is, etching for forming the mesa structure is not performedto the depth of the multi-quantum well (MQW) structure. Thus, thecrystallinity of the multi-quantum well structure is not degraded. Aside surface of the light-receiving layer including the multi-quantumwell structure is not exposed at forming the mesa structure, therebyproviding the light-receiving layer exhibiting low dark current.

In addition, the p-n junction is formed at the end of the p-type regionthat is defined by the diffusion of the p-type impurity (Zn) from thecap layer. At the end of the p-type region, the concentration of thep-type impurity is equal to the background concentration of an n-typeimpurity in the light-receiving layer. Upon receiving light, areverse-bias voltage is applied to the p-n junction to extend adepletion layer toward a portion of the light-receiving layer adjacentto the substrate. Light is absorbed in the depletion layer to generateelectron-hole pairs. The size of the extension of the depletion layerfrom the p-n junction is inversely proportional to the concentration ofthe impurity. To allow the depletion layer to extend largely into thelight-receiving layer, the light-receiving layer is not doped. Here, theimpurity concentration of the light-receiving layer is a backgroundconcentration (n-type, 5×10¹⁵ cm⁻³ or less).

In the light-receiving device according to the first aspect of thepresent invention, the cap layer preferably includes a p-type contactlayer doped with the p-type impurity and a concentration adjustinglayer, the concentration adjusting layer being not doped or being dopedwith a lower concentration of an impurity than the p-type contact layer.The concentration adjusting layer is preferably disposed between thep-type contact layer and the light-receiving layer. The electrode ispreferably disposed on the p-type contact layer.

In the light-receiving device according to the first aspect of thepresent invention, the p-type contact layer may be formed of oneselected from an InGaAs layer and an InP layer. The concentrationadjusting layer may be formed of at least one selected from an InGaAslayer and an InP layer. The p-type region may have a concentration ofthe p-type impurity of 5×10¹⁶ cm⁻³ or less at the boundary between thelight-receiving layer and the cap layer disposed on the light-receivinglayer.

In this case, it is possible to inhibit the degradation of thecrystallinity of the light-receiving layer having the multi-quantum well(MQW) structure due to a predetermined concentration or more of theimpurity.

In the light-receiving device according to the first aspect of thepresent invention, the light-receiving layer may include an undopedtype-II multi-quantum well structure. Furthermore, the type-IImulti-quantum well structure of the light-receiving layer may includeInGaAs layers and GaAsSb layers alternately stacked that is formed onthe substrate made of InP. Alternatively, the type-II multi-quantum wellstructure of the light-receiving layer may include GaSb layers and InAslayers alternately stacked. In this case, it is possible to provide alight-receiving device that is sensitive to the near-infrared regionhaving a wavelength of about 3 μm to the mid-infrared region and thathas low-dark-current characteristics.

A method for producing a light-receiving device according to a secondaspect of the present invention includes the steps of (a) growing alight-receiving layer on a substrate, the light-receiving layer havingan undoped multi-quantum well structure; (b) growing a cap layer on thelight-receiving layer, the cap layer including a p-type semiconductorlayer doped with a p-type impurity; (c) forming a mesa structure byetching the cap layer, the mesa structure being defined by a trenchsurrounding the mesa; (d) after the step of forming the mesa structure,forming a protective film on an upper surface and a side surface of themesa structure; and (e) after the step of forming the protective film,forming a p-n junction in the light-receiving layer or at the boundarybetween the light-receiving layer and the cap layer by annealing withthe upper surface and the side surface of the mesa structure coveredwith the protective film at a predetermined temperature. In the step offorming the mesa structure, the trench reaches the vicinity of an uppersurface of the light-receiving layer. In the step of forming the p-njunction, the p-type impurity in the p-type semiconductor layer isdiffused from the cap layer in the mesa structure to the light-receivinglayer.

In the method according to the second aspect of the present invention,the cap layer including the p-type semiconductor layer doped with thep-type impurity is formed on the light-receiving layer by epitaxialgrowth. Furthermore, the cap layer is etched to form the mesa structure.In the subsequent step of forming the p-n junction, the p-type impurityis diffused from the p-type semiconductor layer in the cap layer towardthe light-receiving layer by annealing at a predetermined temperature.

When an impurity is introduced into an epitaxial wafer from an externalgas phase like selective diffusion, the pile-up of the impurity occursat a heterointerface with the light-receiving layer, thereby causing thespike-like local change of the concentration. In contrast, in theforegoing method, the p-type impurity doped in the p-type semiconductorlayer of the cap layer during epitaxial growth is diffused. This doesnot cause the pile-up of the impurity at the heterointerface between thecap layer and the light-receiving layer.

In the method for a light-receiving device according to the secondaspect of the present invention, preferably, the cap layer includes aconcentration adjusting layer formed on the light-receiving layer and ap-type contact layer formed on the concentration adjusting layer, thep-type contact layer being doped with the p-type impurity. Preferably,the concentration adjusting layer is not doped or is doped with a p-typeor an n-type impurity at a lower concentration than that of the p-typecontact layer. In the step of forming the p-n junction, the p-typeimpurity is preferably diffused from the p-type contact layer in themesa structure to the light-receiving layer through the concentrationadjusting layer.

In the method for a light-receiving device according to the secondaspect of the present invention, preferably, in the step of growing thecap layer, the p-type semiconductor layer in the cap layer is grownwhile the p-type impurity is doped with a concentration gradually orstepwise increased with the lapse of growth time from the beginning ofthe growth. This prevents the degradation of the crystallinity of themulti-quantum well (MQW) structure due to the diffusion of the impuritywith a concentration more than a predetermined concentration.

In the method for a light-receiving device according to the secondaspect of the present invention, preferably, the light-receiving layerand the cap layer are grown at a growth temperature of 425° C. to 575°C. by a metal-organic vapor phase epitaxy method using metal-organiccompounds for a III group source material and a V group source material.

The semiconductor layer constituting the MQW structure such as theGaAsSb layer in the light-receiving layer may cause phase separation ata high temperature to degrade the satisfactory crystallinity. Thus, theGaAsSb layer is grown using only the metal-organic sources, which arereadily decomposed at a low temperature, at 425° C. to 575° C., therebyproviding satisfactory crystallinity and preventing a reduction in thecrystallinity of the GaAsSb layer.

A method for producing a light-receiving device according to a thirdaspect of the present invention includes the steps of (a) growing alight-receiving layer on a substrate, the light-receiving layer havingan undoped multi-quantum well structure; (b) forming a selective growthmask on the light-receiving layer, the selective growth mask includingan opening through which the light-receiving layer is exposed; (c)selectively growing a concentration adjusting layer and a p-type contactlayer, in that order, on the light-receiving layer using the selectivegrowth mask, the p-type contact layer being doped with a p-typeimpurity; and (d) forming a p-n junction in the light-receiving layer orat the boundary between the light-receiving layer and the concentrationadjusting layer. The concentration adjusting layer is not doped or isdoped with a p-type or an n-type impurity at a lower concentration thanthat of the p-type contact layer. In the step of forming the p-njunction, the p-type impurity doped in the p-type contact layer isdiffused to the light-receiving layer through the concentrationadjusting layer during growing the p-type contact layer. In this method,the edge of the p-n junction is not exposed to the atmosphere during theproduction. Furthermore, the p-n junction is separated by using theselective growth, thereby circumventing damage to walls of the mesastructure due to etching. It is thus possible to surely suppress thedark current of the light-receiving device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a light-receiving device and asensing apparatus according to a first embodiment.

FIG. 2A is a partially enlarged view of a pixel of the light-receivingdevice illustrated in FIG. 1. FIG. 2B illustrates the in-depthdistribution of impurity concentration in the light-receiving deviceillustrated in FIG. 1.

FIG. 3A is an enlarged view of a pixel in a modified embodiment of thelight-receiving device illustrated in FIG. 1. FIG. 3B illustrates thein-depth distribution of impurity concentration in the light-receivingdevice illustrated in FIG. 3A.

FIGS. 4A to 4D illustrate a process for producing the light-receivingdevice illustrated in FIG. 1. FIG. 4A illustrates a state in which afteran insulating film is formed on a stacked semiconductor layer, a resistmask is formed on the insulating film and subjected to dry etching. FIG.4B illustrates a state during the dry etching. FIG. 4C illustrates astate in which after an insulating film is formed on the entire surfaceof a wafer including a mesa trench, a resist mask having an opening atan end portion of a chip is formed and subjected to dry etching. FIG. 4Dillustrates a state in which the resist mask and the insulating filmlocated at the end portion of the chip are removed.

FIG. 5 illustrates the in-depth distribution of Zn obtained by aselective diffusion method in the related art, and, in particular,illustrates the pile-up of Zn at a hetero-interface.

FIG. 6 illustrates a light-receiving device and a sensing apparatusaccording to a second embodiment.

FIG. 7A is a partially enlarged view of a pixel of the light-receivingdevice illustrated in FIG. 6. FIG. 7B illustrates the in-depthdistribution of impurity concentration in the light-receiving deviceillustrated in FIG. 6.

FIG. 8A is a cross-sectional view of a light-receiving device and asensing apparatus according to a third embodiment. FIG. 8B is apartially enlarged view of a pixel.

FIGS. 9A and 9B are a cross-sectional view and a plan view,respectively, of a light-receiving device according to a fourthembodiment.

FIGS. 10A to 10D illustrate a process for producing the light-receivingdevice illustrated in FIG. 9. FIG. 10A illustrates a stage at which alight-receiving layer is formed on a buffer layer. FIG. 10B illustratesa stage at which a selective growth mask is formed on thelight-receiving layer. FIG. 10C illustrates a stage at which selectivegrowth layers are grown while an impurity is selectively diffused. FIG.10D illustrates a stage at which pixel electrodes are formed on theselective growth layers.

FIG. 11 is a plan view of the opening pattern of the selective growthmask illustrated in FIG. 10B.

FIG. 12A is a schematic diagram of p-type regions and p-n junctions, thep-type regions extending from selectively grown contact layers servingas diffusion sources. FIG. 12B is a partial enlarged view of edges ofthe p-n junctions illustrated in FIG. 12A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Findings of the present invention can be readily understood byconsidering the following detailed description with reference to theattached drawings illustrated. A light-receiving device, a method forproducing the same, and a sensing apparatus according to embodiments ofthe present invention will be described with reference to the attacheddrawings. The same components are designated using the same referencenumerals, when possible.

First Embodiment

FIG. 1 illustrates a sensing apparatus 50 including a light-receivingdevice 10 and a read-out integrated circuit (ROIC) 70 connected to thelight-receiving device 10. The light-receiving device 10 has a stackedsemiconductor layer described below, (InP substrate 1/n-type InP bufferlayer 2/light-receiving layer 3/cap layer 5). The light-receiving layer3 includes an undoped multi-quantum well (MQW) structure in which welllayers and barrier layers are alternately stacked. Especially, thelight-receiving layer 3 includes an undoped type-II multi-quantum well(MQW) structure. The light-receiving layer 3 including the type-IImulti-quantum well (MQW) structure is explained hereinafter.

The cap layer 5 includes a p-type InGaAs contact layer 5 a and an InGaAsconcentration adjusting layer 5 b adjacent to the p-type InGaAs contactlayer 5 a. A pixel electrode 11 is formed on the p-type InGaAs contactlayer 5 a and is in ohmic contact with the p-type InGaAs contact layer 5a. Zinc (Zn) doped in the p-type InGaAs contact layer 5 a as a p-typeimpurity diffuses into the InGaAs concentration adjusting layer 5 b. Inthe InGaAs concentration adjusting layer 5 b, zinc (Zn) impurity isdistributed so as to have a concentration gradient decreasing toward thelight-receiving layer. The concentration adjusting layer 5 b may beformed of the foregoing InGaAs layer, an InP layer, or a composite layerformed of (InGaAs layer/InP layer). The p-type contact layer 5 a may beformed of the foregoing InGaAs layer, an InP layer, or a composite layerformed of (InGaAs layer/InP layer). The p-type contact layer 5 a has aconcentration of a p-type impurity, such as zinc (Zn), of 1×10¹⁸ cm⁻³ ormore so as to form an ohmic contact with the pixel electrode 11.Impurity concentrations in the p-type contact layer 5 a and theconcentration adjusting layer 5 b will be described in detail below inconnection with a p-n junction 15.

The light-receiving layer 3 has a multi-quantum well (MQW) structure.Specifically, the light-receiving layer 3 has a type-II multi-quantumwell (MQW) structure in which InGaAs and GaAsSb layers are alternatelystacked. Each of the InGaAs layers has a thickness of 2 nm to 6 nm. Eachof the GaAsSb layers has a thickness of 2 nm to 6 nm. The multi-quantumwell structure has about 250 pairs to about 500 pairs. Thelight-receiving layer 3 has a thickness of 2 μm to 5 μm. The p-typecontact layer Sa of the cap layers 5 has a thickness of 0.3 μm to 3 μm.The concentration adjusting layer 5 b has a thickness of 0.05 μm to 2μm. As illustrated in a second embodiment, the concentration adjustinglayer 5 b may be absent.

Pixels P are mechanically and structurally separated by a trench K of amesa structure. To protect an edge portion of the mesa structure, aprotective film 25 is formed on a top surface and a side surface of themesa structure. The protective film 25 is also formed in the trench K ofthe mesa structure. The trench K of the mesa structure, the p-typecontact layer Sa, and a ground electrode 12 are located at the edgeportion. The trench K of the mesa structure, the p-type contact layer 5a, and the ground electrode 12 are covered with the protective film 25.Each of the pixel electrodes 11 of the light-receiving device 10 iselectrically connected to a corresponding read-out electrode 71 providedin the ROIC 70 with a bump (not illustrated). A ground electrode 12 isformed on the n-type buffer layer 2 and is in ohmic contact with then-type buffer layer 2. Furthermore, the ground electrode 12 iselectrically connected to a ground electrode 72 with a bump (notillustrated) via a wiring electrode 13 which is arranged on a sidesurface of the edge portion and which extends to the top surface of thep-type contact layer 5 a. The bump is arranged so as to face the groundelectrode 72 of the ROIC 70. The pixel electrode 11 is composed of AuZn.The ground electrode 12 is composed of AuGeNi. The pixel electrode 11and the ground electrode 12 may be composed of Pt—Ti—Au. As describedabove, the p-type contact layer 5 a has an impurity concentration of1×10¹⁸ cm⁻³ or more. The n-type buffer layer 2 may be doped with Siserving as an n-type impurity in an amount of 1×10¹⁸ cm⁻³ or more. Whenthe pixels are two-dimensionally arranged, the backside of the InPsubstrate 1 may be defined as an incident surface. To increaseresponsivity, an antireflection coating 27 composed of, for example,SiON, may be arranged on the backside of the InP substrate 1. Acomplimentary metal-oxide semiconductor (CMOS)-containing multiplexer isused as the ROIC 70.

In the light-receiving device 10 according to this embodiment, acombination of the mesa structure and the impurity diffusion achievesthe pixel separation and the pixel arrangement having a high fillfactor. When the p-n junction is formed, the edge of the p-n junction isarranged in the light-receiving layer or at the upper boundary of thelight-receiving layer (the upper surface of the light-receiving layer)without exposure to an atmosphere for a moment from the beginning of theproduction to the completion of a product. The edge of the p-n junction15 is not exposed to the atmosphere from the beginning of the productionto the completion of a product; hence, an impurity, such as oxygen, doesnot adhere to the edge of the p-n junction 15. No adhesion of oxygen orthe like to the edge of the p-n junction 15 effectively serves tosuppress dark current. Due to the position of the p-n junction, adepletion layer extends to the light-receiving layer 3 at a lowoperating voltage. The low operating voltage also contributes assuredlyto suppress the dark current.

The mesa structure of the light-receiving device 10 according to thisembodiment will be described below. As illustrated in FIG. 1, the trenchK reaches the vicinity of the upper surface of the light-receiving layer3. Regarding the trench of the mesa structure described above, theexpression “the trench K reaches the vicinity of the upper surface ofthe light-receiving layer” indicates that when the depth position of theboundary between the light-receiving layer 3 and an upper layer (theconcentration adjusting layer 5 b in each of the cap layers 5 in FIG. 1)on the light-receiving layer 3 is defined as a reference depth position,the bottom of the trench is located at a position in a depth range of (aposition in the upper layer away from the reference depth position by1/10 of the thickness of the light-receiving layer 3) to (a position inthe light-receiving layer 3 away from the reference depth position by2/10 of the thickness of the light-receiving layer 3). Thus, the bottomof the trench K is located at a position in the concentration adjustinglayer 5 b of a corresponding one of the cap layers 5 away from thereference depth position by 10% or less of the thickness of thelight-receiving layer 3. Alternatively, when the bottom of the trench Kis located in the light-receiving layer 3, the depth position of thebottom of the trench K is limited to the inside of an upper portion ofthe light-receiving layer 3, the upper portion having a thickness of 20%or less of the thickness of the light-receiving layer 3. Accordingly, anear-infrared light-receiving device which includes the pixels Pseparated mechanically and structurally is obtained. In addition, in thenear-infrared light-receiving device, excellent performances such as lowdark current and high signal-to-noise ratio can be realized. Withrespect to pixel separation in the related art, the trench is arrangedso as to extend substantially completely through the light-receivinglayer. As a result, a side wall of the light-receiving layer is exposedto an atmosphere at the trench. Therefore, in the light-receiving devicewith the trench extending completely through the light-receiving layer,dark current is increased. In the embodiment, the trench K is formed soas to have the bottom which is located at a position in the upper layeraway from the reference depth position by 10% or less of the thicknessof the light-receiving layer or which are each located at a position inthe upper portion of the light-receiving layer 3 away from the referencedepth position by 20% or less of the thickness of the light-receivinglayer 3. Therefore, the pixels P are separated without exposure of thep-n junction 15 to the outside air. The mesa structure is formed byforming the trench K surrounding the mesa structure, and then the mesastructure is covered with the protective film 25. An impurity (Zn) isdiffused to form the final p-n junction 15. Specifically, the trench Kis formed within the depth range described above. In the embodiment, ap-type impurity (Zn) doped in the p-type contact layers 5 a is diffusedtoward the light-receiving layer 3. The p-type contact layers 5 a isused as a diffusion source.

A method for forming the p-n junction 15 by impurity diffusion from thep-type contact layer 5 a will be described below. In a planar-typelight-receiving device using the selective diffusion of an impurity inthe related art, pixels are separated by regions in which the impuritydoes not diffuse. In the selective diffusion, however, the impurity isdiffused from opening portions not only in the depth direction but alsoin the lateral direction. This is a major obstacle to reducing the pixelpitch to increase the fill factor. In contrast, according to thisembodiment, pixels are mechanically and structurally separated by themesa structure, thereby resulting in a narrow pitch. According to thefirst embodiment, after forming the protective film 25 on the sidesurface and top surface of the mesa structure, the p-type impurity, suchas Zn, in the p-type contact layers 5 a is diffused by annealing to theupper surface of the light-receiving layer 3 or inside thelight-receiving layer 3. Thus, the p-n junction 15 is accurately locatedat predetermined positions on the upper surface of the light-receivinglayer 3 or in the upper portion of the light-receiving layer 3.

FIG. 2A is a partially enlarged cross-sectional view of the pixel Pillustrated in FIG. 1 and illustrates the distribution of Zn serving asa p-type impurity in a pixel P mechanically and structurally separatedby the trench K of the mesa structure. FIG. 2B illustrates the in-depthdistribution of the concentration of the impurity. Zinc (Zn) serving asa p-type impurity is diffused from the p-type contact layer 5 a towardthe light-receiving layer 3 by a driving force resulting from aconcentration gradient, thereby forming a p-type region 6. An end of thep-type region 6 is located at a position which is inside thelight-receiving layer 3 and in the vicinity of the upper surface of thelight-receiving layer 3, thereby resulting in the formation of the p-njunction 15.

Referring to FIG. 2B, immediately after the epitaxial growth of thep-type contact layer 5 a, the impurity is uniformly distributed(indicated by a dotted line) with high concentration in the p-typecontact layer 5 a. In the annealing process, the impurity is diffusedtoward the light-receiving layer 3. As a result, the impurityconcentration at a corner portion of a high-concentration region in thep-type contact layer 5 a is gradually reduced toward the light-receivinglayer 3. In the concentration adjusting layer 5 b disposed between thep-type contact layer 5 a and the light-receiving layer 3, a distributionin Zn concentration has a large slope of (concentration/in-depthdistance) in the depth direction. As described above, the concentrationof the p-type impurity, such as Zn, may be set at a low concentration of5×10¹⁶ cm⁻³ or less at the boundary between the light-receiving layer 3and the concentration adjusting layer 5 b because the concentrationadjusting layer 5 b is disposed between the p-type contact layer 5 a andthe light-receiving layer 3. This inhibits the degradation of thecrystallinity of the type-II multi-quantum well structure constitutingthe light-receiving layer 3.

The p-n junction 15 is formed at a position (crossing position) at whichthe concentration of the p-type impurity at the end of the p-type region6 is equal to the background concentration of an n-type impurity in thelight-receiving layer 3. When the position of the p-n junction 15 isdefined as a reference position, the p-type region 6 extending from thereference position to the p-type contact layer 5 a has a high p-typeimpurity concentration. The light-receiving layer 3 is composed of anundoped semiconductor that has an n-type conductivity and a backgroundconcentration of the n-type impurity. Therefore, the concentration ofthe n-type impurity in a portion of the light-receiving layer 3extending from the reference position toward the substrate 1 is equal tothe background concentration. The background concentration of the n-typeimpurity in the portion of the light-receiving layer 3 extending fromthe reference position toward the substrate 1 is, for example, about5×10¹⁵ cm⁻³ and is substantially constant. The p-type impurity in thep-type contact layer 5 a, which serves as a diffusion source, is dopedduring growing the contact layer 5 a. The p-type contact layer 5 aserves as a diffusion source and the p-type impurity in the p-typecontact layer 5 a is diffused toward the light-receiving layer 3. In themethod, steps of vacuum-sealing an epitaxial wafer in a silica tube orthe like and selectively diffusing an impurity from the outside througha gas phase are not required. It is thus possible to improve theproductivity by increasing the diameter of the semiconductor substrate.

When the light-receiving device 10 receives light, it is necessary toapply a predetermined operating voltage (the absolute value of areverse-bias voltage) to the p-n junction 15 to extend a depletion layerinto the light-receiving layer 3. As illustrated in FIG. 2, the p-njunction 15 is formed in the light-receiving layer 3 or at the boundarybetween the light-receiving layer 3 and the concentration adjustinglayer 5 b in the cap layer 5. In this case, even if a low reverse-biasvoltage is applied to the light-receiving device 10, the depletion layerextends into the light-receiving layer 3.

As described above, in general, dark current is proportional to anoperating voltage (the absolute value of a reverse-bias voltage). Thus,a lower operating voltage results in further suppression of the darkcurrent. As illustrated in FIG. 2B, the portion of the light-receivinglayer 3 extending from the reference position of the p-n junction 15toward the substrate 1 has a low impurity concentration that correspondsto the n-type background concentration. On the other hand, a portion ofthe light-receiving layer 3 extending from the reference position towardthe concentration adjusting layer 5 b contains a p-type impurity with ahigh concentration. Thus, by applying a low operating voltage, thedepletion layer is extended largely into the light-receiving layer 3.The operating voltage is applied between the pixel electrode 11 and theground electrode 12. In this embodiment, the depletion layer issufficiently extended in the light-receiving layer even at a lowoperating voltage, so that sufficient responsivity is provided. Asdescribed above, the reduction in operating voltage results in thesuppression of the dark current.

Modified Embodiment

FIGS. 3A and 3B illustrate a modified embodiment of the light-receivingdevice 10 illustrated in FIG. 1. FIG. 3A is a partially enlargedcross-sectional view of the pixel P. FIG. 3B illustrates the in-depthdistribution of the concentration of Zn serving as a p-type impurity.The difference between the modified embodiment and the light-receivingdevice 10 illustrated in FIGS. 1 and 2 is that a trench K is deepened insuch a manner that the bottom of the trench K is located at a position 3b in the light-receiving layer 3. When the trench K is deepened to thelight-receiving layer 3, the position of the bottom is located at aposition in the light-receiving layer 3 away from the boundary betweenthe concentration adjusting layer 5 b and the light-receiving layer 3 by2/10 or less of the thickness of the light-receiving layer. Asillustrated in FIG. 3A, the deep mesa trench K increases a region wherethe depth position of the p-n junction 15 lies at a shallower depth thanthe depth position of the bottom of the mesa trench. In this case, thepixels P can be more surely separated from adjacent pixels.

A method for producing the light-receiving device 10 will be describedbelow. As illustrated in FIG. 4A, the n-type InP buffer layer 2, thelight-receiving layer 3 having a type-II multi-quantum well structure,the concentration adjusting layer 5 b including an InP layer and/or anInGaAs layer, and the p-type InGaAs contact layers 5 a are grown, inthat order, on the InP substrate 1 by an epitaxial growth method to forma stacked semiconductor layer. The light-receiving layer 3 includes atype-II multi-quantum well structure. The concentration adjusting layer5 b includes an InP layer and/or an InGaAs layer. As the epitaxialgrowth method, for example, a metal-organic vapor phase epitaxy (MOVPE)method using metal-organic compounds for a III group source material anda V group source material may be employed. For example, as a V groupsource material, tertiarybutylarsine (TBAs) and tertiarybutylphosphine(TBP) are used. TBAs and TBP are source materials of As and P,respectively. In the MOVPE method, low-temperature growth may beperformed because the metal-organic sources are decomposed at relativelylow temperatures. It is thus possible to inhibit the decomposition andso forth of the type-II multi-quantum well structure during growing thestacked semiconductor layer. Hence, the foregoing semiconductor layersare grown by the MOVPE method to form a light-receiving layer havingsatisfactory crystallinity. In the MOVPE method, as a p-type-impuritysource or the like, a metal-organic source may also be used. Forexample, diethylzinc (DEZn), which is a metal-organic material, may beused as a Zn source. The growth temperature may be set at a temperaturebetween 425° C. and 575° C. At the growth temperature, the crystallinityof the type-II MQW structure composed of, for example, InGaAs and GaAsSbis not degraded. As the InP substrate 1, an Fe-doped semi-insulating InPsubstrate is used because of a low optical absorption of the Fe-dopedsemi-insulating InP in the near-infrared region. Alternatively, aconductive InP substrate may be used, depending on a production process.The impurity concentration in the p-type contact layer 5 a is 1×10¹⁸cm⁻³ or more. The n-type buffer layer 2 is doped with Si at aconcentration of 1×10¹⁸ cm⁻³ or more. The concentration adjusting layer5 b may be lightly doped with an n-type impurity or a p-type impurity ina concentration of 1×10¹⁶ cm⁻³ or less. Alternatively, the concentrationadjusting layer 5 b may not be doped.

A trench of a mesa structure is formed to separate the pixels P in apixel region. To form the trench of the mesa structure, an insulatingfilm 21 is formed on the stacked semiconductor layer. The insulatingfilm 21 is formed of, for example, SiO₂ or SiN. A mask R1 composed of aresist is formed on the insulating film 21. Arrows in FIGS. 4A and 4Bindicate etching positions. The insulating film 21 is etched withbuffered hydrofluoric acid using the mask R1 to form an etching mask. Asillustrated in FIG. 4B, the stacked semiconductor layer is etched by dryetching using the patterned insulating film 21 as the etching mask. As aresult of etching the stacked semiconductor layer, the trench K of themesa structure is formed in the pixel region. In this dry etching, it ispossible to determine the time at which the dry etching is stopped whenthe bottom of the trench K reaches the upper surface of thelight-receiving layer by monitoring antimony in the GaAsSb layerincluded in the type-II MQW structure. Then the insulating film 21 isremoved. As described above, when the depth position of the boundarybetween the light-receiving layer and the upper layer (the concentrationadjusting layer 5 b in this embodiment) on the light-receiving layer isdefined as the reference depth position, the depth of the trench K ofthe mesa structure is in the range of (a position in the upper layeraway from the reference depth position by 1/10 of the thickness of thelight-receiving layer) to (a position in the light-receiving layer awayfrom the reference depth position by 2/10 of the thickness of thelight-receiving layer). Then a damaged layer is removed by wet etching.

Next, a groove G for the formation of the ground electrode 12 is formedat the edge portion. In this embodiment, the depth of the groundelectrode groove G is significantly different from the depth of thetrench K for pixel separation. To form the groove G and the trench K,the stacked semiconductor layer is thus etched in two steps. Asillustrated in FIG. 4C, an insulating film 22 is formed on the entiresurface. The insulating film 22 is formed of, for example, SiO₂ or SiN.In the trench K for the pixel separation, side surfaces of the mesastructures are also surely covered and protected. This protectionprevents damage to the side surfaces of the mesa structures due to thedry etching, thereby achieving low dark current. A resist mask R2 isformed so as to have an opening at an edge of the light-receiving device10. As illustrated in FIG. 4D, the ground electrode groove G for theformation of the ground electrode 12 is formed by dry etching. Thereby,the ground electrode groove G is formed at the edge of thelight-receiving device 10. In FIG. 4D, the resist mask R2 is omitted.

In FIG. 1 and so forth, the bottom and the inner walls of the trench Kand top surfaces of the stacked semiconductor layer are protected by theprotective film 25. That is, the side surfaces and upper surfaces of themesa structures are protected by the protective film 25. Regarding theprotective film 25, the insulating film 22 illustrated in FIG. 4D) isused, as-is, as the protective film 25.

Next, an annealing process is performed by heating to 400° C. to 550° C.with the bottom and the inner walls of the trench K and top surfaces ofthe stacked semiconductor layer covered with the protective film 25.Accordingly, the annealing process is performed with the side surfacesand the upper surfaces of the mesa structures covered with theprotective film 25. In the annealing process, the p-type impurity (Zn)doped in the p-type contact layer 5 a is diffused toward thelight-receiving layer 3. In the embodiment, after the mesa structure iscovered with the protective film, the p-type impurity is diffused toform the p-n junction 15 at the upper surface of the light-receivinglayer 3 or in the light-receiving layer 3. The p-n junction 15 is notexposed to an atmosphere in a growth chamber during the annealingprocess (in which p-type impurity is diffused). Hence, an impurity, suchas oxygen, is not attached to the edge of the p-n junction 15, leadingto a reduction in leakage current. Therefore, an increase in the darkcurrent of the light-receiving device 10 can be suppressed.

Next, a mask composed of a resist is formed in order to form the pixelelectrode 11. The protective film 25 is etched with the resist mask toform an opening for forming the electrode on the mesa structure. Ap-side electrode composed of, for example, AuZn is formed as the pixelelectrode 11 by, for example, a lift-off process on the upper surface ofthe mesa structure. An n-side electrode to be formed into the groundelectrode 12 common to the pixels is formed. The n-type electrode may becomposed of, for example, AuGeNi. The wiring electrode 13 extending fromthe ground electrode 12 to the protective film 25 on the mesa structurethrough a surface of the n-type InP buffer layer 2 and a wall of thestacked semiconductor layer is formed. The backside of the InP substrate1 is polished to a thickness of about 100 μm. The antireflection (AR)coating 27 as illustrated in FIG. 1 is formed on the backside of the InPsubstrate 1.

In the embodiment, as a method for forming an impurity region, such asthe p-type region 6, the impurity doped in the impurity layer, such asthe p-type contact layer 5 a is diffused. In addition, the p-n junction15 is formed by the diffusion of the impurity (Zn) from the impuritylayer, such as the p-type contact layer 5 a. Here, the impurity (Zn) isdoped in the impurity layer during growing the impurity layer. It isthus possible to obtain an impurity concentration distribution differentfrom the case where an impurity is introduced from the outside gasphase. FIG. 5 illustrates the in-depth distribution of Zn obtained by aselective diffusion method in the related art. The in-depth distributionof Zn shown in FIG. 5 is obtained by the selective diffusion methoddescribed below. At first, a stacked semiconductor layer including anInP buffer layer, a light-receiving layer, an InGaAs layer, and an InPwindow layer is formed on an InP substrate. The InP buffer layer, thelight-receiving layer, the InGaAs layer, and the InP window layer aregrown in this order on the InP substrate by, for example, MOVPE. The InPwindow layer is a top layer of the stacked semiconductor layer. Thelight-receiving layer includes a type-II multi-quantum well structure. Aselective diffusion mask having openings above the InP window layer isformed to provide an intermediate product (epitaxial wafer). Theintermediate product (epitaxial wafer) is vacuum-sealed in a silica tubetogether with a solid zinc source. The zinc source sublimes by heattreatment. The gas-phase Zn that has sublimed diffuses in desiredregions (pixel regions) in the epitaxial wafer from the gas phasethrough the openings of the selective diffusion mask to form Zn regions.As a result, the in-depth concentration distribution as illustrated inFIG. 5 is obtained when zinc (Zn) is selectively diffused from theoutside gas phase through the openings of the selective diffusion maskon the epitaxial wafer.

When a large amount of the diffusion source is present in the gas phase,compared with that in the epitaxial wafer or the light-receiving layer,a small spike-like Zn peak is observed at the heterointerface betweenInGaAs and the (InGaAs/GaAsSb) type-II multi-quantum well structure, asdescribed as “Pile-up of Zn” in FIG. 5. This is referred to as the“pile-up of Zn”.

In contrast, in the embodiment, the method is employed in which theimpurity region, such as p-type region 6, is formed using the diffusionfrom the p-type contact layer 5 a containing the p-type impurity (Zn)serving as a diffusion source. In the method according to theembodiment, the pile-up of Zn as illustrated in FIG. 5 does not occur atthe heterointerfaces between the concentration adjusting layers 5 b andthe light-receiving layer 3. Thus, the absence of the pile-up of animpurity (Zn) indicates that the diffusion source of the impurity is thep-type contact layers 5 a.

Second Embodiment

FIG. 6 illustrates the sensing apparatus 50 in which the light-receivingdevice 10 according to a second embodiment is connected to the read-outintegrated circuit (ROIC) 70. The difference from the light-receivingdevice 10 according to the first embodiment is that the cap layer 5 isformed of the p-type contact layer 5 a alone without the concentrationadjusting layer in this embodiment. The bottom of the trench K of themesa structure is located at a position away from the upper surface ofthe light-receiving layer 3 by 20% or less of the thickness of thelight-receiving layer 3.

FIG. 7A is a partially enlarged view of a pixel P illustrated in FIG. 6.FIG. 7A also illustrates a cross-sectional view of the distribution ofZn serving as a p-type impurity in the pixel P. In the embodiment, thepixels P are mechanically and structurally separated from each other bythe trench K of the mesa structure. FIG. 7B illustrates the in-depthdistribution of Zn concentration. In the pixel P, zinc (Zn) is directlydiffused from the p-type contact layer 5 a into the light-receivinglayer 3 without a concentration adjusting layer by a driving forceresulting from a concentration gradient. The p-type region 6 extends tothe light-receiving layer 3. The p-n junction 15 is formed at the end ofthe p-type region 6. The concentration of the p-type impurity is 5×10¹⁶cm⁻³ or less at the boundary between the light-receiving layer 3 and thep-type contact layer 5 a serving as an upper layer on thelight-receiving layer 3, thereby maintaining satisfactory crystallinityof the type-II InGaAs/GaAsSb multi-quantum well structure. However, thep-type contact layer 5 a is a region with which the pixel electrode 11forms an ohmic contact. Thus, at least a surface of the p-type contactlayer 5 a needs to have a p-type impurity concentration of 1×10¹⁸ cm⁻³or more. To achieve both the two points concerning the impurityconcentration, when the p-type contact layer 5 a is epitaxially grown,the doping amount of the p-type impurity may be gradually or stepwiseincreased from an undoped state or a lightly doped state with the lapseof growth time from the beginning of the growth. For example, asillustrated in FIG. 7B, a step-like concentration distribution indicatedby a dotted line may be obtained immediately after the growth of thep-type contact layer Sa by the doping method. In the concentrationdistribution immediately after the growth, the impurity concentration is5×10¹⁶ cm⁻³ or less at the boundary between the light-receiving layer 3and the p-type contact layer 5 a.

Hereafter, the p-type impurity exhibiting the step-like concentrationdistribution is diffused toward the light-receiving layer 3 byannealing. The shape of the step-like concentration distributionimmediately after the growth is changed from a rectangular shape to arounded shape by the diffusion. However, the impurity concentration isnot more than 5×10¹⁶ cm⁻³ at the boundary between the light-receivinglayer 3 and the p-type contact layer 5 a. Thus, there is no degradationin the crystallinity of the type-II multi-quantum well structure due toa high impurity concentration.

Third Embodiment

FIGS. 8A and 8B illustrate the light-receiving device 10 according to athird embodiment and the sensing apparatus 50 including thelight-receiving device 10. FIG. 8A is a cross-sectional view of thesensing apparatus. FIG. 8B is a partially enlarged view of a pixel P.Structures of the light-receiving device 10 according to this embodimentand the sensing apparatus 50 are different from the light-receivingdevice 10 according to the first embodiment and the sensing apparatus 50as illustrated in FIG. 1 and so forth in the following points:

(1) walls of the trench K of the mesa structure are inclined. This mesashape may be obtained by forming the mesa structure by wet etching;(2) the bottom of the trench K of the mesa structure is located in theconcentration adjusting layer 5 b of the cap layer 5. The bottom of thetrench K is located at a position in the concentration adjusting layer 5b away from the upper surface of the light-receiving layer 3 by 10% orless of the thickness of the light-receiving layer 3; and(3) the p-n junction 15 is formed at the boundary between thelight-receiving layer 3 and the concentration adjusting layer 5 b.

The light-receiving device 10 in this embodiment also provides the sameadvantageous effects as the foregoing first embodiment. The sameproduction method may also be employed, except that the mesa structureis etched by wet etching.

The formation of the trench K and the groove G by wet etching does notsubstantially cause damage to the crystal. Thus, the removal of adamaged layer by wet etching may not be performed or may be performed.

Fourth Embodiment

FIGS. 9A and 9B are a cross-sectional view and a plan view,respectively, of a light-receiving device according to a fourthembodiment. A plurality of pixels P are provided on the InP substrate 1.To detect optical signals independently one another, the plural pixels Pare separately arranged. Referring to FIGS. 9A and 9B, thelight-receiving device 10 has a stacked semiconductor layer describedbelow, (InP substrate 1/buffer layer 2/type-II MQW light-receiving layer3/cap layer 5).

The n-type buffer layer 2 may be composed of InP, InAlAs, or InGaAs.Light is incident on the backside of the InP substrate 1. To increasethe amount of light received, the antireflection coating 27 composed ofSiON or a multilayer film is disposed on the backside of the InPsubstrate 1.

The InP substrate 1 is composed of an Fe-doped semi-insulating InP. Theground electrode 12 is arranged on the n⁺-type buffer layer 2. Theundoped type-II MQW light-receiving layer 3 is grown withoutintentionally doping any impurities. The type-II MQW light-receivinglayer 3 contains an n-type impurity, such as Si, in a concentration ofbackground of 1×10¹⁶ cm⁻³ or less. The cap layer 5 may be composed ofInGaAs or InP. The cap layers 5 are selectively grown within openings 25h of the selective growth mask 25. Each of the cap layers 5 includes thep-type contact layer 5 a and the concentration adjusting layer 5 b. Thep-type contact layer 5 a in the cap layer 5 is doped with a p-typeimpurity (for example, Zn). The p-side electrode 11 serving as the pixelelectrode is arranged on the p-type contact layer 5 a. The p-sideelectrode 11 is composed of, for example, AuZn. The n-side electrode 12which is composed of AuGeNi and which serves as the ground electrode isarranged on the n⁺-type buffer layer 2.

According to the plan view of FIG. 9B, the pixels P are arranged in therespective openings 25 h of the selective growth mask 25 in plan.Regarding the arrangement of the pixels P, for example, the pixel pitchis 30 μm. The length of a side of each square opening 25 h is 20 μm to25 μm. The pixel P is a unit of the light-receiving element. Asillustrated in FIG. 9B, the pixels P are two-dimensionally arrayed.Thus, for example, imaging may be performed. While the openings 25 hhave a rectangular or square shape in plan view in the foregoingembodiment, they may have a circular shape or the like.

In this embodiment, the cap layers 5 are selectively grown within theopenings 25 h of the selective growth mask 25. The mechanical skeletonof the pixels P is formed with the cap layers 5. The selective growthmask 25 is formed so as to be in contact with the light-receiving layer3. Then the cap layers 5 (5 b and 5 a) are selectively grown on thelight-receiving layer 3 through the openings 25 h of the selectivegrowth mask 25 to have a substantially square shape in plan view andthus a post-shape in side view. A semiconductor layer is not grown onthe selective growth mask 25. The selective growth mask 25 is composedof, for example, SiN or SiO₂. Thus, the cap layers 5 (5 b and 5 a) areselectively grown in the respective openings 25 h. A production methodaccording to this embodiment does not include a step of forming a mesastructure or a step of performing etching a mesa. In the light-receivingdevice according to this embodiment, gaps between the cap layers 5correspond to a mesa trench. The skeleton of the pixels P is defined bythe cap layers 5 (5 b and 5 a) confined to the openings 25 h.

The formation of a p-n junction according to this embodiment will bedescribed below. The pixel P includes the p-n junction or pi junction toextend a depletion layer to the light-receiving layer 3. Furthermore,the p-n junction is used for separating the pixels P from each other. Inthis embodiment, when the cap layer 5 (5 b and 5 a) is epitaxiallygrown, zinc (Zn) is added as a p-type dopant. The growth temperature is450° C. or higher. Zn is thermally diffused toward the light-receivinglayer 3 during the growth of the cap layer 5. The light-receiving layer3 contains a low concentration of the n-type impurity as describedabove. Thus, Zn is diffused from the bottom of the cap layer 5 towardthe light-receiving layer 3 while the p-type region 6 is being formed.In this way, the p-n junction 15 may be formed at a position near theupper surface of the light-receiving layer 3 or the boundary between thelight-receiving layer 3 and the cap layer 5 (the concentration adjustinglayer 5 b).

When the light-receiving device is operated, a reverse-bias voltage isapplied between the n-side electrode 12 and the p-side electrode 11 togenerate an electric field at the p-n junction 15. At this time, in thelight-receiving device according to this embodiment, a depletion layeris formed so as to extend from the p-n junction 15 toward alow-impurity-concentration region, i.e., a region containing a lowconcentration of the n-type impurity. In other words, the depletionlayer extends toward the lower surface of the light-receiving layer 3.When light is incident on the backside of the InP substrate 1, light isabsorbed in the depletion layer. At this time, electron-hole pairs areefficiently formed in the depletion layer. Among the resultingelectron-hole pairs, electrons move to the n-side electrode 12, and theholes move to the p-side electrode 11, thereby accumulating charges inproportion to the amount of light received. Thus, the optical signal isefficiently converted into an electrical signal. The light-receivingdevice supplies the electrical signal. Providing the electrical signalin response to the intensity of light received for each pixel P resultsin the intensity distribution of light received, thereby enablingimaging or the like. The separation of the p-n junctions 15 fromadjacent pixels P will be described in detail in the section of aproduction method. In the description, the suppression of dark currentwill also be described.

FIGS. 10A to 10D schematically illustrate a method for producing thelight-receiving device 10 illustrated in FIGS. 9A and 9B. As illustratedin FIG. 10A, the n⁺-type buffer layer 2 and the undoped light-receivinglayer 3 having a type-II MQW structure are grown by an epitaxial growthmethod on the Fe-doped semi-insulating InP substrate 1. The n⁺-typebuffer layer 2 is composed of, for example, InP, InAlAs, or InGaAs. Forthe epitaxial growth, for example, a metal-organic vapor phase epitaxymethod using only metal-organic sources may be employed withmetal-organic compounds for a group III source and a group V source. AsGa (gallium), In (indium), As (arsenic), P (phosphorus), and Sb(antimony) sources, triethylgallium (TEGa), trimethylindium (TMIn),tertiarybutylarsine (TBAs), tertiarybutylphosphine (TBP), andtrimethylantimony (TMSb) are used, respectively. As the Ga source,trimethylgallium (TMGa) may also be used. As the In source,triethylindium (TEIn) may be used. As the As source, trimethylarsine(TMAs) may also be used. As the Sb source, triethylantimony (TESb) mayalso be used. As an n-type dopant, tetraethylsilane (TeESi) may be used.As a p-type dopant, diethylzinc (DEZn) may be used. In the metal-organicvapor phase epitaxy method using only metal-organic sources, a size of ametal-organic vapor molecule in the source is large, and themetal-organic vapor molecule is decomposed at a low temperature to growa semiconductor layer. Thus, in the metal-organic vapor phase epitaxymethod using only metal-organic sources, a semiconductor layer may begrown even at a relatively low temperature. The number of InGaAs—GaAsSbpairs in the MQW structure is in the range of about 50 to 500.

Next, the selective growth mask 25 is formed on the light-receivinglayer 3. The selective growth mask 25 is formed of a dielectric filmcomposed of, for example, SiN, SiON, or SiO₂. FIG. 11 illustrates thepattern of the selective growth mask 25. As described above, the lengthof a side of each square opening 25 h is 20 μm to 25 μm. Each of thelongitudinal and transverse pitches of the openings 25 h is about 30 μm.The light-receiving layer 3 is exposed through the openings 25 h of theselective growth mask 25. Comparing FIG. 11 and FIG. 9B reveals that theopenings 25 h correspond to central regions of the pixels P in plan.FIG. 10B illustrates a state in which the selective growth mask 25 isformed. Then the cap layers 5 each including the p-type contact layer 5a and the concentration adjusting layer 5 b are selectively grown on thelight-receiving layer 3 through the openings 25 h using the selectivegrowth mask 25. No growth occurs on a region covered with the selectivegrowth mask 25. When the p-type contact layer 5 a in the cap layer 5 isformed, for example, diethylzinc (DEZn), which is an organometalliccompound, may be used as a dopant source of a p-type impurity. The caplayer 5 including the p-type contact layer 5 a and the concentrationadjusting layer 5 b may be composed of, for example, InGaAs, InP, or acomposite layer of InGaAs and InP. The p-type contact layers 5 a and theconcentration adjusting layers 5 b may be composed of the same materialor different materials. However, the p-type contact layer Sa is dopedwith the p-type impurity in a high concentration. On the other hand, theconcentration adjusting layer 5 b is doped with the n-type impurity orthe p-type impurity in a low concentration. Alternatively, theconcentration adjusting layer 5 b may be not doped (undoped). Thepresence of the concentration adjusting layers 5 b enables theadjustment of the concentration of the p-type impurity that is diffusedfrom the p-type contact layer 5 a toward the light-receiving layer 3.

The concentration adjusting layers 5 b are selectively grown thoroughthe selective growth mask 25 on the light-receiving layer 3 exposed atthe openings 25 h. At this time, nothing is grown on a region of theselective mask other than the openings. The growth temperature of theconcentration adjusting layers Sb and the contact layers 5 a is 450° C.or higher and 550° C. or lower. At the growth temperature, thecrystallinity of the type-II MQW structure is not degraded. Furthermore,the satisfactory crystallinity of the selective growth layers, such asthe concentration adjusting layers 5 b, is maintained. Moreover, at thegrowth temperature, DEZn is decomposed into Zn serving as a p-typeimpurity. It is thus possible to dope the contact layers 5 a with theimpurity, Zn, in a high concentration during the growth of the contactlayers Sa. In addition, the growth temperature is sufficient to allowthe impurity, Zn, with which the contact layers 5 a are doped to diffusefrom the bottoms of the contact layers 5 a into the light-receivinglayer 3 while the concentration adjusting layers 5 b are changed to ap-type region.

At the foregoing growth temperature, the Zn concentration at a positionnear the upper surface of the light-receiving layer 3 can be adjusted toabout 5×10¹⁵ cm⁻³, which is the background concentration of the n-typeimpurity in the light-receiving layer 3, by adjusting the thickness ofthe concentration adjusting layers 5 b. Thereby, the p-n junction 15 isarranged at a desired position in the light-receiving layer 3. FIG. 10Cillustrates a state in which the p-n junction 15 is formed in thelight-receiving layer 3. Note that an edge M of the p-n junction islocated on the surface of the light-receiving layer 3. The p-typeimpurity is diffused from the contact layer 5 a through theconcentration adjusting layer 5 b in the thickness direction (depthdirection or vertical direction). The p-type impurity is also diffusedin the transverse direction (direction along a layer surface). As aresult, each of the p-type regions 6 is slightly larger than acorresponding one of the openings 25 h in plan.

The edge M of the p-n junction 15 is located on the surface of thelight-receiving layer 3. Here, the edge M of the p-n junction 15 iscovered with the region of the selective growth mask 25 other than theopening, i.e., the dielectric film. Thus, an impurity, such as oxygen,in the atmosphere is not attached to the edge M of the p-n junction. InFIG. 10C, the range of a pixel P is demarcated by a thick broken line.The range of each p-type region 6 in plan corresponds to a correspondingone of the pixels P. Then the p-side electrode 11 is formed on thep⁺-type contact layer Sa. The ground electrode (not illustrated) isformed on the n⁺-type buffer layer 2. This leads to the completion ofthe light-receiving device 10 illustrated in FIG. 9A. The selectivegrowth mask 25 is left as is in the product because it functions as apassivation film.

Next, dark current of the light-receiving device produced in theproduction method according to this embodiment will be described. Whenthe edge M of the p-n junction 15 is exposed to the atmosphere, oxygenand so forth in the atmosphere are attached to cause an increase in darkcurrent. It is thus necessary to prevent the edge M of the p-n junction15 from being exposed to the atmosphere or an atmosphere in a growthchamber even during the production. In this embodiment, as illustratedin FIG. 10C, Zn is diffused from the p⁺-type contact layer 5 a to thelight-receiving layer 3 through the concentration adjusting layer 5 b.As illustrated in FIG. 12B, the edge M appears on the surface of thelight-receiving layer 3 at the stage in which the p-n junction 15 isformed in the light-receiving layer 3. At this time, the edge M of thep-n junction 15 has already been covered with the dielectric film or theregion of the selective growth mask 25 other than the opening. Theselective growth mask 25 is left as a passivation film in thelight-receiving device, so that the edge M of the p-n junction 15 is notexposed to the atmosphere or the like hereafter. As a result, the edge Mof the p-n junction 15 is not exposed to the atmosphere or theatmosphere in the growth chamber, thereby providing the light-receivingdevice having low-dark-current characteristics.

Regarding the independence of the pixels P, the skeleton of the pixels Pis formed with the discrete cap layers 5 as described above.Furthermore, the p-n junctions 15 are separated from the p-n junctions15 of adjacent pixels P, so that adjacent pixels P are surely andelectrically separated from each other. As illustrated in FIG. 12B, thediffusion of the p-type impurity to the transverse direction may besubstantially negligible, compared with diffusion in the verticaldirection (thickness direction or depth direction). Thus, when apredetermined level of the pitch of the cap layers 5 or the pitch of theopenings 25 h is ensured, adjacent pixels P are surely and electricallyseparated from each other.

The crystallinity of the light-receiving layer of the light-receivingdevice produced by the production method according to this embodimentwill be described below. When the light-receiving layer has a type-IIMQW structure, the MQW structure is not so stable, compared with a bulkcrystal. For example, when the type-II MQW structure is processed at atemperature equal to or higher than a predetermined temperature or thetype-II MQW structure is heavily doped at a high impurity concentration,the MQW structure may be changed. Alternatively, the layer structure ofthe MQW structure may be decomposed. With respect to the temperature,the temperature at which the cap layers 5 a and 5 b are grown may be setto a relatively low temperature of, for example, 600° C. or lower andeven 550° C. or lower. For example, by using the metal-organic vaporphase epitaxy method using only metal-organic sources, a semiconductorlayer may be grown even at a relatively low temperature. With respect tothe impurity concentration, the thickness of the undoped orlightly-doped concentration adjusting layer 5 b is adjusted in such amanner that an excess of the p-type impurity is not diffused to thelight-receiving layer 3 during the growth of the cap layer 5, inparticular, the p-type contact layers 5 a. In other words, the undopedor lightly-doped concentration adjusting layer 5 b is used as a layerconfigured to adjust the diffusion concentration distribution of thep-type impurity. In this case, the concentration of Zn in eachconcentration adjusting layer 5 b decreases monotonically from the sideof a corresponding one of the contact layers 5 a (diffusion source)toward the light-receiving layer 3. Note that the pile-up of Zn at theheterointerface does not occur.

With respect to the type-II MQW structure, (InGaAs/GaAsSb), (GaSb/InAs),and so forth may be used. A (InGaAs/GaAsSb) type-II MQW structure isformed on an InP substrate. A (GaSb/InAs) type-II MQW structure isformed on a substrate selected from GaAsSb substrates, GaAs substrates,and InP substrates.

When the concentration adjusting layer 5 b is provided, the foregoingstructure results in a reduction in the concentration of the p-typeimpurity in the light-receiving layer 3. Unlike the structure accordingto this embodiment, even when the concentration adjusting layer 5 b isnot provided, a doping method may be employed in which, for example, inthe step of forming the p-type contact layer 5 a, the doping amount ofthe p-type impurity is gradually or stepwise increased from an undopedstate or a lightly doped state with the lapse of growth time. The amountof the p-type impurity diffused toward the light-receiving layer 3 canbe adjusted and reduced by the doping method of the impurity in the caseof the absence of the concentration adjusting layer. Thus, even in thecase of the absence of the concentration adjusting layer, it is possibleto form the light-receiving layer 3 having a satisfactory type-II MQWstructure with a high crystal quality. Naturally, in the case of thepresence of the concentration adjusting layer 5 b, the foregoing dopingmethod may also be employed. Furthermore, when the selective growthlayer is composed of a semiconductor material, such as InGaAs, having arelatively high electrical conductivity even at a low impurityconcentration, it is possible to suppress the disadvantage of increasingthe electrical resistance or the like even if a layer with a lowconcentration of the p-type impurity is included in the cap layer 5.

While the embodiments and the examples of the present invention havebeen described, the disclosed embodiments and examples of the presentinvention are intended to illustrate and not limit the scope of theinvention. The scope of the present invention is defined by the Claims.All changes which fall within meanings and scopes equivalent to theClaims are included.

1-8. (canceled)
 9. A method for producing a light-receiving device, themethod comprising the steps of: growing a light-receiving layer on asubstrate, the light-receiving layer having an undoped multi-quantumwell structure; growing a cap layer on the light-receiving layer, thecap layer including a p-type semiconductor layer doped with a p-typeimpurity; forming a mesa structure by etching the cap layer, the mesastructure being defined by a trench surrounding the mesa; after the stepof forming the mesa structure, forming a protective film on an uppersurface and a side surface of the mesa structure; and after the step offorming the protective film, forming a p-n junction in thelight-receiving layer or at the boundary between the light-receivinglayer and the cap layer by annealing with the upper surface and the sidesurface of the mesa structure covered with the protective film at apredetermined temperature, wherein, in the step of forming the mesastructure, the trench reaches the vicinity of an upper surface of thelight-receiving layer, and in the step of forming the p-n junction, thep-type impurity in the p-type semiconductor layer is diffused from thecap layer in the mesa structure to the light-receiving layer.
 10. Themethod according to claim 9, wherein the cap layer includes aconcentration adjusting layer formed on the light-receiving layer and ap-type contact layer formed on the concentration adjusting layer, thep-type contact layer being doped with the p-type impurity, theconcentration adjusting layer is not doped or is doped with a p-type oran n-type impurity at a lower concentration than that of the p-typecontact layer, and in the step of forming the p-n junction, the p-typeimpurity is diffused from the p-type contact layer in the mesa structureto the light-receiving layer through the concentration adjusting layer.11. The method according to claim 9, wherein, in the step of growing thecap layer, the p-type semiconductor layer in the cap layer is grownwhile the p-type impurity is doped with a concentration gradually orstepwise increased with the lapse of growth time from the beginning ofthe growth.
 12. The method according to claim 9, wherein thelight-receiving layer and the cap layer are grown at a growthtemperature of 425° C. to 575° C. by a metal-organic vapor phase epitaxymethod using metal-organic compounds for a III group source material anda V group source material.
 13. The method according to claim 9, whereinthe light-receiving layer includes an undoped type-II multi-quantum wellstructure.
 14. A method for producing a light-receiving device, themethod comprising the steps of: growing a light-receiving layer on asubstrate, the light-receiving layer having an undoped multi-quantumwell structure; forming a selective growth mask on the light-receivinglayer, the selective growth mask including an opening through which thelight-receiving layer is exposed; selectively growing a concentrationadjusting layer and a p-type contact layer, in that order, on thelight-receiving layer using the selective growth mask, the p-typecontact layer being doped with a p-type impurity; and forming a p-njunction in the light-receiving layer or at the boundary between thelight-receiving layer and the concentration adjusting layer, wherein theconcentration adjusting layer is not doped or is doped with a p-type oran n-type impurity at a lower concentration than that of the p-typecontact layer, and in the step of forming the p-n junction, the p-typeimpurity doped in the p-type contact layer is diffused to thelight-receiving layer through the concentration adjusting layer duringgrowing the p-type contact layer.